Field of the Invention
The invention relates to the field of high voltage circuit interruption in electrical devices such as switchgear, transformers, and the like, and in particular concerns high voltage current limiting fuses or expulsion fuses, circuit breakers, circuit interrupters, separable cable connectors, or the like, including a pulverulent arc-quenching filler material of high dielectric strength that is adapted to aid in arc extinction, and to quickly and effectively to break the circuit. More particularly, the invention is directed to an arc-quenching filler material encased within a high voltage current limiting device that is surface modified with a gas-evolving composition to provide improved arc-quenching properties without impairing the free flowing and compacting properties of the arc-quenching filler material. The invention also concerns a method of making the same.
Current limiting power interruption in high voltage circuits requires a current interruption device that rapidly and effectively brings the current to a zero value upon the occurrence of a line fault. The fuse devices generally considered herein are those employed in electrical circuits typically at voltages of a thousand or more volts. Electrical circuits operating at such high levels of voltage can cause extensive damage to circuit components, machinery connected to the circuit, or the like if the current interruption is not accomplished positively in a short period following the occurrence of fault or overload conditions.
Expulsion fuses or gas-evolving fuses in particular have been used extensively for high voltage circuit interruption in switchgear, transformers, and other electrical equipment. It is generally known that arc-quenching and gas-evolving materials in such a circuit interruption device, positioned in contact with the fuse element, aid in, inter alia, deionizing, cooling, and thereby quenching the electric arc created under fault or overload current conditions.
It is known to provide a pulverulent (powder) arc-quenching filler material, for example sand, inside the, casing of a fuse to absorb the energy of a burning or fusing fuse element during the fusing process so that the fuse will not explode when interrupting the circuit. The conventional arc-quenching filler material tends to confine the arc radially and thus to sustain its current limiting voltage, in addition to absorbing the energy of the arc. However, such fuse when operating under low current conditions may arc for an extended period if time during which the sand or powdered arc-quenching filler may be heated sufficiently to be fused. In the fused state, the conventional arc-quenching filler suffers a loss in insulation properties which can be sufficient to prevent interruption of the current or to allow a restrike after a temporary interruption. It has been difficult to obtain, however, an arc-quenching filler material that is substantially resistant to a fused state, thereby forming fulgurites.
It is also known to provide mandrels or cores of gas-evolving materials to evolve an arc-quenching gas during the fusing operation. To avoid excessive pressures against the inside of the fuse housing and ferrules which may lead to rupture of the fuse housing or blow off the ferrules, the amount of evolved gas can be reduced by locally positioning gas-evolving materials in controlled small quantities along the core. The pressure within the fuse housing does not, therefore, increase unduly, and the positive effects of the presence of arc suppressing gas are generally maintained. It has been difficult to obtain, however, a gas-evolving material whose solid residue in the fused state is relatively non-conductive, so as to prevent restriking or tracking of the arc by conductance through the fused compound, and a tendency to reestablish a current flow through the material after interruption.
A typical high voltage fuse can include a generally tubular casing of electrically insulating material; a pair of terminal elements closing each of the opposite ends of the casing; a pulverulent arc-quenching filler material of high dielectric strength inside the casing, such as sand, mica beads, or finely divided quartz; a fuse element or elements made of a highly conductive material, such as silver, submersed in the filler and conductively interconnecting the terminal elements, the fuse element or elements typically being wound in a parallel-connected relationship along the length of a supporting mandrel or core; a core of high dielectric strength electrically insulating high temperature material, such as ceramic, the core providing support for the fuse element or elements and having longitudinally and radially extending fins of a cross-shaped, star-shaped or like cross-section, along the longitudinal axis of the casing; and a gas-evolving material regionally distributed along the length of the core in contact with the fuse element or elements.
In operation, when the high voltage current limiting fuse is subjected to an applied current that exceeds the rated current carrying capability of the fuse element, the excessive current causes sufficient resistive heating that the fuse element attains a fusion temperature. Melting and vaporization of the fuse element occur at one or more predetermined locations along its length, whereupon an electrical arc is established in each region where the fuse element melts. A plurality of series connected arcs can be formed along the fuse element. Current limitation occurs when the sum of the individual arc voltages reaches the voltage applied to the fuse. Thus, the current limiting effect results from the introduction of arc resistance in series with the circuit.
When electrical arcing occurs, the fuse element and/or its metal vapors rapidly expand to many times the volume originally occupied by the fuse element. These metal vapors expand into the spaces between portions of the arc-quenching filler material where they condense through heat transfer into the arc-quenching filler, and consequently are no longer positioned for current conduction. The physical contact between the hot arc and the relatively cool arc-quenching filler granules causes a rapid transfer of heat from the arc to the granules to dissipate most of the arc energy without substantial pressure buildup within the fuse casing. A material that rapidly evolves a deionizing gas may be distributed along the length of the core to reduce conduction through gas that may be ionized by the arc and to cool the arc, which facilitates arc extinction under low current conditions.
However, after this fusing operation occurs, fulgurites are formed in the pulverulent arc-quenching filler material. That is, the pulverulent arc-quenching filler material is fused or sintered in the hot arcing regions into a glass-like body defining a path of relatively lower resistance than the surrounding pulverulent material. The fulgurites provide a path along which restrike of the arc current can occur. There is a need to provide a high voltage current limiting device that uses the beneficial properties of energy-absorbing pulverulent arc-quenching filler material and localized evolvement of arc-suppressing gas while at the same time reducing the tendency to form conductive fulgurites in the fusing region.
A typical arc-extinguishing gas-evolving material may comprise a combination of a gas-evolving material and a thermoplastic or thermosetting polymeric structural binder. Such material generally is highly carbonizing and therefore conductive. Upon gas evolution, the organic binder decomposes, leaving conductive carbon residues. There is a need to provide a high voltage current limiting device that uses the properties of energy-absorbing pulverulent arc-quenching filler material and localized evolvement of arc-suppressing gas while reducing the tendency to form carbon residues in the fusing region. Carbon residue likewise enhances the opportunity for a restrike of the arc, which is undesirable.
U.S. Pat. No. 4,099,153 (Cameron) teaches a high voltage current limiting fuse comprising a fuse element wrapped about an electrically insulating support mandrel or core along the core length, the fuse element being held in position on the core by gas-evolving C-clamps locally distributed along the length of the core. The core, fuse element, and gas-evolving clamps are embedded in a pulverulent arc-quenching filler inside a casing. Cameron teaches positioning the gas-evolving clamps in contact with the fuse element in localized regions. Upon fusing and arcing, the pressure of the evolved gas forces the arc-quenching filler away form the restricted arcing regions. Cameron claims that this reduces formation of fulgurites in those regions during fusing, so that undesirable restriking of the arc will not occur.
U.S. Pat. No. 4,319,212 (Leach) teaches a high voltage current limiting fuse comprising a fuse element wrapped about a finned core with cutouts along its length, and with gas-evolving materials positioned in the cutouts. The core, fuse element, and the gas-evolving material are surrounded by a granular arc-quenching filler material inside a casing. Leach teaches positioning the arc-quenching pulverulent filler in the immediate vicinity of the arc-initiating fuse element. The filler absorbs the arc energy as the fuse element melts, and forms fulgurites which Leach claims are cooled and rendered insulating, rather than conductive, by evolved gases also in close proximity to the arc-initiating fuse element melts and the arc-quenching filler material.
U.S. Pat. No. 3,582,586 (Jones) teaches a gas-evolving material comprising melamine and a thermoplastic or thermosetting organic binder. As discussed above, such gas-evolving material has a tendency to carbonize in air under arcing conditions to form conductive carbon residues which enhances arc restriking and tracking.
U.S. Pat. No. 3,761,660 (Jones) teaches a gas-evolving material comprising melamine, hydrated alumina and a thermoplastic or thermosetting organic binder. The hydrated alumina is provided to release the water of its hydration to enhance arc-quenching properties and to catalyze the oxidation of carbonaceous materials to reduce carbon residue formation. A drawback of hydrated materials in a current limiting device is the tendency to cause corrosion as a result of evolution of water from the hydrated material, and ionization during arcing.
U.S. Pat. No. 4,975,551 (Syverston) teaches a gas-evolving material comprising of melamine or other related compounds containing carboxylic reactive groups, such as amine, hydroxyl, epoxy, aziridine, or thiol groups, and a thermoplastic polymer containing carboxylic acid moieties which chemically bond to the melamine or related compounds carboxylic acid reactive group. Carboxylic acid moieties are highly carbonizing in their fused state and, consequently, have a tendency to track the arc.
It would be desirable to provide a pulverulent arc-quenching filler material that has its surfaces modified with a relatively non-carbonizing gas-evolving material that can be used in a high temperature current limiting device to rapidly and effectively quench an arc. It would be further desirable to provide a pulverulent arc-quenching filler material modified with a relatively non-carbonizing gas-evolving material that maintains the free flowing and compacting characteristics of the pulverulent arc-quenching filler material. It would also be desirable to provide a pulverulent arc-quenching filler material modified with a relatively non-carbonizing gas-evolving material that tends to quench the follow current, i.e., the current which flows through the hot fulgurite after a fusing operation, through cooling of the fulgurites by the evolved gas. The evolved gas of such gas-evolving material on the surface of the arc-quenching filler material advantageously produces a deionizing action on the arc initiated by vaporization of the fuse element, and reduces the tendency for a restrike or track of the arc by reducing fulgurite formation and/or cooling the fulgurite formed to a more insulating and less conductive body. Such modified arc-quenching filler material can be provided in direct contact with the fuse element.